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Optical and Infrared Detectorsfor Astronomy
Basic Principles to State-of-the-art
James W. Beletic
NATO/ASI and Euro Summer School
Optics in AstrophysicsSeptember 16-27, 2002
Goals of the Detector Course
The student should gain an understanding of:
3. The role detectors play in an astronomical observatory
2. Fundamental detector physics
3. Standard detector architecture
4. What affects quantum efficiency and readout noise
5. The state-of-the-art today
6. Special applications / areas of research & development
Why detectors are the MOST important technology!
Course Outline
Lecture 1: Role of detectors in observatoryDetector physicsStandard architecture
Lecture 2: Quantum efficiencyReadout noiseDetector imperfections
Lecture 3: Manufacturers Bigger devices / MosaicsElectronicsSpecial applicationsOptical CMOS and CMOS + CCD
Optical and Infrared Astronomy(0.3 to 25 m)
Telescope to collect and focus light
Instrument
Instrument to measure light
Two basic parts
Telescope to collect and focus light
Instrument to measure light
Optical and Infrared Astronomy(0.3 to 25 m)
Okay, maybe a bit more complicated – 4 basic parts
Telescope to collect and focus light
AdaptiveOptics
Detector
Instrument tomeasure light
Optics
Instrument goal is to measure a 3-D data cube
But detectors are 2-dimensional !• Our detectors are• Can not measure color, only intensity
So the optics of the instrument are used to map a portion of the 3-D data cube on to the 2-D detector
BLACK & WHITEWHITE
Wav
eleng
th
Declination
Right Ascension
Intensity
Where detectors are used in an observatory
Scientific: ImagingSpectroscopy
Technical: Acquisition / guidingActive opticsAdaptive opticsInterferometry (fringe & tip/tilt tracking)Site monitoring (seeing, clouds, LGS)
General: SurveillanceSafety
Detector zoology
X-ray Visible NIR MIR
λ [µm]
Silicon CCD & CMOS0.3 1.
10.9 2.5 5 20
HgCdTe
InSb
STJ
0.1
Si:As
In this course, we concentrate on 2-D focal plane arrays. • Optical – silicon-based (CCD, CMOS)• Infrared – IR material plus silicon CMOS multiplexer
Will not address: APD (avalanche photodiodes)
STJs (superconducting tunneling junctions)
The Ideal Detector
• Detect 100% of photons
• Each photon detected as a delta function
• Large number of pixels
• Time tag for each photon
• Measure photon wavelength
• Measure photon polarization
Up to 99% quantum efficiency
One electron for each photon
over 355 million pixels
No - framing detectors
No – defined by filter
No – defined by filter
Plus READOUT NOISE and other “features”
5 basic steps of optical/IR photon detection
1. Get light into the detectorAnti-reflection coatings
2. Charge generationPopular materials: Silicon, HgCdTe, InSb
3. Charge collectionElectrical fields within the materialcollect photoelectrons into pixels.
4. Charge transfer
If infrared, no charge transfer required.
For CCD, move photoelectrons to the edgewhere amplifiers are located.
5. Charge amplification & digitization
Amplification process is noisy. In general
CCDs have lowest noise, CMOS and IRdetectors have higher noise.
Quantum
Efficiency
Point
Spread
Function
Se
nsitv
ity
Take notice
• Optical and IR focal plane arrays are similar in many ways– I will combine information about optical and IR
detectors as much as possible.
• But optical and IR detectors are different in some important ways– I will try to be careful to differentiate when necessary.
• Please ask if you are ever confused whether the subject is optical and/or IR detectors.
Step 1: Get light into the detectorAnti-reflection coatings
• AR coatings will be discussed in lecture 2 when quantum efficiency is presented.
Step 2: Charge Generation
Photon Detection
Conduction Band
Valence Band
Eg
For an electron to be excited from theconduction band to the valence band
h Eg
h = Planck constant (6.6 10-34 Joule•sec) = frequency of light (cycles/sec) = /c
Eg = energy gap of material (electron-volts)
240.05Si:AsArsenic doped Silicon
5.90.23 InSbIndium Antimonide
1.24 – 141.00 – 0.09HgCdTeMer-Cad-Tel
1.11.12SiSilicon
c ( m)Eg (eV)SymbolMaterial Name
c = 1.238 / Eg (eV)
Tunable BandgapA great property of Mer-Cad-Tel
Hg1-xCdxTe
Modify ratio of Mercury and Cadmium
to “tune” the bandgap energy
1.7.730.55
1.241.0 0.7
5.250.295
3.410.395
10.120.21
14.090.196c ( m)Eg (eV)x
Step 2: Charge Generation
Photon Detection
Conduction Band
Valence Band
Eg
For an electron to be excited from theconduction band to the valence band
h Eg
h = Planck constant (6.6 10-34 Joule•sec) = frequency of light (cycles/sec) = /c
Eg = energy gap of material (electron-volts)
4240.05Si:AsArsenic doped Silicon
305.90.23 InSbIndium Antimonide
20 - 801.24 – 141.00 – 0.09HgCdTeMer-Cad-Tel
163 - 3001.11.12SiSilicon
Operating Temp. (K)c ( m)Eg (eV)SymbolMaterial Name
c = 1.238 / Eg (eV)
How small is an electron-volt (eV) ?
1 eV = 1.6 • 10-19 J
1 J = N • m = kg • m • sec-2 • m
1 kg raised 1 meter = 9.8 J = 6.1 • 1019 eV
How small is an electron-volt (eV) ?DEIMOS example
DEIMOS – Deep Extragalactic Imager
& Multi-Object Spectrograph• 8K x 8K CCD array – 67 million pixels• If 100 images / night, then ~13.5 Gbyte/night• If used 1/3 of the year & all nights clear, 1.65 Tbyte/year• If average pixel contains 5,000 photoelectrons
4.1 • 1015 photoelectons / year4.6 • 1015 eV / year
Single peanut M&M candy (2 g) falling 15 cm (6 inches) loses potential energy equal to 1.85 • 1016 eV, same as total bandgap energy from four years of heavy DEIMOS use.
Step 3: Charge Collection
• Intensity image is generated by collecting photoelectrons generated in 3-D volume into 2-D array of pixels.
• Optical and IR focal plane arrays both collect charges via electric fields.
• In the z-direction, optical and IR use a p-n junction to “sweep” charge toward pixel collection nodes.
2-D arrayof pixelsy
zx
Photovoltaic Detector Potential Well
Silicon CCD & HgCdTe and InSb are photovoltaic detectors. They use a pn junction to generate E-field in the z-direction of each pixel. This electric
field separates the electron-hole pairs generated by a photon.
Note bene !
Can collect eitherelectrons or holes
For silicon
n – region from phosphorous doping
p – region fromboron doping
n-channel CCDcollects electrons
p-channel CCDcollect holes
Step 3: Charge Collection
• Optical and IR focal plane arrays are different for charge collection in the x and y dimensions.
• IR – collect charge at each pixel and have amplifiers and readout multiplexer
• CCD – collect charge in array of pixels. At end of frame, move charge to edge of array where one (or more) amplifier (s) read out the pixels.
2-D arrayof pixelsy
zx
Infrared Detector Cross-section(InSb example)
epoxy
MOSFET inputindium bump bond
implanted p-type InSb(collect holes)
AR coating
Thinned bulk n-type InSb
silicon multiplexer
OutputSignal
IncidentPhotons
Infrared Detector Cross-section(new Rockwell HgCdTe design)
One StepGrowth
Buried JunctionIntersection
ActiveAbsorber
Layer
N-MWIR MCT
P-Type Implant
Thin Film CdTeExternal Passivant
CdZnTe Substrate
Metal
MCTCap Layer
IncidentPhotons
(collect holes)
For silicon
n – region from phosphorous doping
p – region fromboron doping
n-channel CCDcollects electrons
p-channel CCDcollect holes
Step 4: Charge transfer
• IR detectors have amplifier at each pixel, so no need for charge transfer.
• CCDs must move charge across the focal plane array to the readout amplifier.
CCD Charge transferThe good, the bad & the ugly
• “Bad & ugly” aspects of charge transfer– Takes time– Can blur image if no shutter used– Can lose / blur charge during move– Can bleed charge from saturated pixel up/down column– Can have a blocked column– Can have a hot pixel that releases charge into all passing pixels
• “Good” aspects of charge transfer– Can bin charge “on-chip” – noiseless process– Can charge shift for tip/tilt correction or to eliminate systematic
errors (“va-et-vient”, “nod-and-shuffle”)– Can build special purpose designs that integrate different areas
depending on application (curvature wavefront sensing, Shack-Hartmann laser guide star wavefront sensing)
– Can do drift scanning
– Have space to build a great low noise amplifier !
Step 5: Charge amplification
• Similar for CCDs and IR detectors.• Both use MOSFETs (metal-oxide-silicon field
effect transistors) to amplify the signal.• Show CCD amplifier first and then relate to IR
pixel.
Amplifier Responsivity(SITe example)
Q = CV
V = Q / C
Capacitance of MOSFET = 10-13 F (100 fF)
Responsivity of amplifier = 1.6 V / e-
More recent amplifier designs have higher responsivity, 5 – 10 V/e-, which give lower noise, but less dynamic range. Research is being done on 50 xx amplifier designs which may lead to sub-electron read noise.
Infrared Detector Cross-section(InSb example)
epoxy
MOSFET inputindium bump bond
implanted p-type InSb(collect holes)
AR coating
Thinned bulk n-type InSb
silicon multiplexer
OutputSignal
IncidentPhotons
IR multiplexer pixel architecture
Vreset
reset voltage
Vdd
amp drain voltage
OutputDetectorSubstrate
PhotvoltaicDetector
Reset
IR multiplexer pixel architecture
Vreset
reset voltage
Vdd
amp drain voltage
OutputDetectorSubstrate
PhotvoltaicDetector
Enable
Reset
“Clock” (green)
“Bias voltage” (purple)
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